1. Technical Field
The present invention relates to a technology for measuring explosives and chemical weapons using high energy laser-induced deuterium neutrons, and, more particularly, to a prompt gamma-ray detection apparatus for analyzing chemical materials using femtosecond pulse laser-induced neutrons, which can be effectively used in the nondestructive inspection of various materials, such as metals, coal, cement, radioactive materials and the like as well as explosives and chemical materials, and which can provide better measurement results for analysis of basic materials, and to a method of measuring prompt gamma-rays using the apparatus.
2. Description of the Related Art
Generally, it is known that a laser-induced neutron generator produces high-energy radioactive rays and fast neutrons by the interaction between high-energy femtosecond pulse lasers and deuterium.
Further, conventionally, Prompt Gamma-ray Neutron Activation Analysis (PGNAA) has been known as a technology of detecting nuclear reaction products by converting fast neutrons (˜2.5 MeV) generated by the D-D nuclear reaction of a neutron generator into thermal neutrons.
As a conventional example of such prompt gamma-ray neutron activation analysis, Korean Patent registration No 10-0935801 (2009.12.29) discloses “a prompt gamma-ray detection system and a discrimination level determination method for detecting prompt gamma-rays using the same”.
That is, the “prompt gamma-ray detection system and discrimination level determination method for detecting prompt gamma-rays using the same” disclosed in the Korean Patent registration No 10-0935801 intends to solve the problem of it being difficult for a conventional prompt gamma-ray detection system to accurately detect a sudden drop in radiation because the background attributable to neutrons is excessively high and to solve the problem of it being difficult to practically use a conventional prompt gamma-ray detection system in clinical radiation treatment although it can be generally used in proton beam measurement because the size of a neutron shielding material is excessively large.
Further, in order to solve the above problems, it is required to develop an optimized discrimination level determination method which can effectively measure prompt gamma-rays and simultaneously can reduce the influence of background gamma-rays by analyzing the energy spectrum of the measured gamma rays instead of decreasing the radiation of background gamma-rays using a shielding material. Therefore, the Korean Patent registration No 10-0935801 intends to provide a prompt gamma-ray detection system that can accurately measure a sudden drop in radiation based on the discrimination level at which the influence of background gamma-rays can be minimized and at which prompt gamma-rays can be effectively measured, and a discrimination level determination method for detecting prompt gamma-rays using the same.
In order to accomplish the above purpose, the prompt gamma-ray detection system of the Korean Patent registration No 10-0935801 comprises: a case including a first region, a second region and a rectangular through-hole penetrating one side of the first region and the other side of the second region; a deceleration unit disposed in the first region and made of paraffin; an absorption unit disposed in the second region and made of boron carbide; a shielding unit disposed in the absorption unit and made of lead; and a measurement unit detecting prompt gamma-rays that have passed through the through-hole and measuring the amount of radiation thereof.
Further, as another conventional example of such prompt gamma-ray neutron activation analysis, Korean Unexamined Patent Publication No. 10-2010-0119194 (2010.11.09) discloses “a non-destructive inspection method for nuclear fuel rod concentration using a pulsed D-D neutron generator”.
Meanwhile, at the time of using a conventional non-destructive inspection apparatus, there are the problems of having to increase the intensity of a neutron source (Cf-252) in order to accomplish high resolution and rapid measurement, having to increase the strength and volume of a shielding in order to block a high radiation dosage rate generated from a neutron source having a high-intensity neutron flux, the neutron source (Cf-252) having to be replaced every three years because its half-life is 2.5 years, it taking a lot of money to maintain this conventional non-destructive inspection apparatus, it being difficult to maintain this conventional non-destructive inspection apparatus, the price of the neutron source (Cf-252) increasing, and the measurement reliability of this conventional non-destructive inspection apparatus decreasing as a result of coefficient compensation depending on the half-life of an isotope (Cf-252). Therefore, in order to solve the above problems, the non-destructive inspection method for nuclear fuel rod concentration using a pulsed D-D neutron generator, disclosed in the Korean Unexamined Patent Publication No. 10-2010-0119194, intends to provide a non-destructive inspection method and apparatus which can measure the concentration of nuclear fissionable materials of a nuclear fuel rod using a pulsed D-D neutron generator and which can rapidly check whether or not a nuclear fuel rod is abnormal.
For this purpose, Korean Unexamined Patent Publication No. 10-2010-0119194 discloses a non-destructive inspection apparatus for inspecting the distribution of concentration of nuclear fuel rods, which includes: a neutron generation unit which intermittently generates neutrons and applies the neutrons to the nuclear fuel rod; and a gamma-ray measurement unit which is disposed under the neutron generation unit with it disposed between the nuclear fuel rods and which measures prompt gamma-rays generated by the nuclear reaction of the nuclear fuel rods.
That is, more concretely, the prompt gamma-ray neutron activation analysis is a method of analyzing the elements in a sample by measuring prompt gamma-rays generated by the nuclear reaction of nuclei and thermal neutrons in the element.
As the neutron radiation source used in such prompt gamma-ray neutron activation analysis, an atomic reactor for research, a radioactive isotope (252Cf) or a small neutron generator may be used.
Here, the atomic reactor is advantageous in that it has a high thermal neutron flux of 1.0×108·cm−2·s−1 or more, but is disadvantageous in that all samples must be moved to the atomic reactor and the size of the sample is limited.
Further, the radioactive isotope is advantageous in that system mobility is easy and in that it is possible to analyze a sample even when the volume of the sample is large, but is disadvantageous in that it is always required to block neutrons because the radioactive isotope spontaneously produces neutrons and in that thermal neutron flux is relatively low.
In contrast, when a femtosecond pulse laser-induced neutron generator is used, a radioactive isotope is advantageous in that system mobility becomes easier and in that it is not required to additionally block neutrons at the time of movement or storage because neutrons are produced only when the laser is being operated.
Further, 252Cf, which is a radioactive isotope, is disadvantageous in that its neutron flux is subject to being continuously decreased because it has a relatively short half-life (2.6 years). However, a femtosecond pulse laser-induced neutron generator using a D-D nuclear reaction is advantageous in that it can control neutron flux by adjusting the intensity of a laser because it can obtain almost a permanent neutron flux.
Therefore, as described above, when the femtosecond pulse laser-induced neutron generator is used as the neutron radiation source used in the prompt gamma-ray neutron activation analysis, the element in the sample can be non-destructively analyzed. Therefore, it is preferred that a prompt gamma-ray detection apparatus for analyzing chemical materials using femtosecond pulse laser-induced neutrons be used. However, to date, a prompt gamma-ray detection apparatus and method completely satisfying such requirements has not yet been provided.